In the propagation of optical pulses over extreme distances, such as, for example, transoceanic distances, numerous parameters associated with such transmission limit the capacity of the optical fiber cable system through which the optical pulses are transmitted and, hence, the amount of intelligence that can be transmitted. The most obvious of such parameters is fiber loss, which degrades and diminishes the pulses passing therethrough. In general, such loss can be compensated by periodic pulse regeneration and amplification. Thus, in an extremely long cable transmission system, spaced regenerators are used for periodically restoring the pulses to a viable condition. Most recently, the use of erbium doped fiber amplification incorporated in the transmitting optical fibers has made it possible to transmit multi-gigabit per second pulse signals over extreme distances, such as transoceanic, without the necessity of regeneration of the signals. Thus, cable or fiber loss can be substantially eliminated as a factor in the transmission of pulse signals, leaving the parameters of fiber dispersion, fiber nonlinearities and amplifier noise as the limiting factors. As discussed in U.S. Pat. No. 5,157,744 of Korotky, optical fibers are not strictly linear, but have a small amount of nonlinearity in their transmission characteristics which make possible a pulse transmission mode in the optical fiber that is effectively immune from the aforementioned pulse degradation factors. As will be discussed more fully hereinafter, this mode, known as 'soliton propagation", effectively balances out the aforementioned factors provided the pulses have a required power level and hence velocity of propagation and a pulse shape in both the time and frequency domains that is optimum for compensating dispersion at a given power level and for reducing interference and cross-talk among pulses. Coupled with erbium doped amplification, which maintains the power required for soliton propagation, it is possible to transmit over large distances with minimum pulse degradation and loss resulting from the aforementioned factors as well as cross-talk. In soliton propagation, there is very little interaction among differing wavelength channels, hence, an additional significant advantage of such propagation is that wavelength division multiplexing (WDM) may be used to increase further the fiber capacity.
Prior art arrangements for generating pulses for use in soliton transmission have most often comprised some form of mode-locked laser. Such a pulse generator produces pulses having a bit rate that is tied to the time of a round trip of the laser cavity. It can be appreciated that extreme precision in sizing the cavity resonator is required for a given frequency, and tunability of repetition rate and pulse width is not easily realized. As a consequence, such a mode-locked laser pulse generator is itself a limiting factor in the transmission system.
The prior art pulse generator as herein discussed may comprise a semi-conductor injection laser that has a continuous wave (CW) non-modulated output. The output is then fed to an optical modulator having a switching characteristic to which is applied a pulse waveform modulating voltage. A Mach-Zehnder interferometric type of modulator has generally been preferred for such systems. However, a high-repetition-rate electrical pulse waveform is difficult to produce and propagate.
The present invention is, therefore, directed towards the reduction or elimination of the use of complex wave forms in an improved soliton pulse generator.